1. Field of the Invention
This invention relates generally to vehicle wheel alignment systems and, more particularly, to image sensors and processors that are used to determine the angles of vehicle wheels and the distances between vehicle wheels.
2. Related Art
Aligning vehicle wheels within specific tolerances is important for optimal control of the vehicle and for consistent wear of the tires. Alignment is performed primarily by adjusting camber, caster, toe, and steering axis inclination. As part of calculating the alignment angles for the vehicle, the angles of the wheels must be determined. The angles can be determined relative to an external reference, such as found in machine vision systems, or relative to the other wheels, such as found in wheel-mounted systems. It is known that these angles can be measured using an electro-optical transducer that incorporates a solid state detector array. In the case of machine vision systems, the detector array may have multiple columns and rows forming an area to capture a two-dimensional image, and in the case of wheel-mounted systems, the detector array may only need to be linear, having a single row with as few as two receptor elements. (In the present application, an “element” may include one or more pixels.) In either case, the image on the detector must be analyzed meticulously so that accurate alignment angles can be calculated.
Wheel-mounted alignment systems typically have sensor heads on each wheel of the vehicle, and each sensor head has an emitter and a receiver that works in combination with at least one other sensor head along the vehicle's sides and across the vehicle. The receiver units may have photodiodes as set forth in U.S. Pat. No. 4,302,104 or a charge coupled device (CCD) as set forth in U.S. Pat. Nos. 5,018,853 and 5,519,489, and the emitter units may have a single source as in U.S. Pat. Nos. 4,302,104 and 5,018,853 or multiple sources as in U.S. Pat. No. 5,488,471. The disclosures of these patents are incorporated herein by reference. Angles and distances are calculated according to the positions of the spots or lines that are detected by the linear arrays.
Machine vision alignment systems typically use a solid state camera with an array detector mounted some distance away from the vehicle to obtain an image of a wheel mounted target. The target incorporates an accurately reproduced pattern that has known control features, as set forth in U.S. Pat. No. 6,064,750, incorporated herein by reference. The position of the features in the image are found and the orientation of the wheel can be calculated by well known algorithms. Some machine vision systems do not use a predefined target but identify particular geometric features on the wheel or tire, such as raised lettering or the circular wheel rim, and use characteristics of the geometric features, such as area, height, width, centoid, corner location, etc., to determine positions and orientations. Co-assigned U.S. patent application Ser. No. 10/439,153, the disclosure of which is incorporated herein by reference, discloses such a system.
In wheel alignment systems, the imaging requirements are somewhat different than a standard camera. Very precise measurements are preferably made at a rate of at least 2 Hz. on static or very nearly static scenes. (Of course, sampling frequencies even slower than 2 Hz could also be used.) This requires stable, low-noise images that have excellent focus and contrast. The accuracy of the measurement depends on the precision with which edges, centroids, corners, lines or boundaries can be determined. Methods for analyzing the image must take into account the possible sources of inaccuracy and compensate for them. To obtain these images, current wheel alignment systems use analog receivers that cannot be integrated onto an application specific integrated circuit (ASIC) with the image processor or the analog to digital converter.
CCD technology has become the dominant method for constructing the solid state receiver arrays. While many alignment systems have been made using CCD elements, the detector has some characteristics that are not ideal for a robust economical product. The CCD element is an expensive component that requires additional support electronics to create a digital output for processing or imaging. It requires a number of timing and control signals as inputs, many of which require different voltages. Supply voltages, clock phases and control signals must be carefully controlled so that extraneous electrical noise is not introduced into the system. The analog output of the CCD element must be converted to a digital format using a separate amplifier and an analog-to-digital converter.
The pixel structure of a CCD element also makes it susceptible to blooming. When light falls on each pixel, photons are converted to electrons which accumulate in the active area of the pixel. If the light is intense or the amount of time the electrons are allowed to accumulate is long, the capacity of the pixel structure to hold the charge will be exceeded. The charge then spills into adjacent pixels and blooming occurs. Most CCD elements have some form of anti-blooming control which minimizes the problem, but it cannot be fully prevented.
There are essentially three different types of CCD structures which may be used in wheel alignment systems, and each type has particular disadvantages. The interline transfer CCD structure has alternating rows or columns of pixels and collectors resulting in a low fill factor and making it susceptible to distortion. Between each row or column of pixels is a row or column for shifting the pixel charge, thereby reducing the photosensitive area to a small percentage of the sensor's total area. This low fill factor may distort intensity profiles, thereby increasing the possibility in machine vision systems that edges and centroids of objects in the image are improperly located. The full frame CCD structure has a high fill factor but requires an external shutter to control the integration time of the device. The extra cost and complexity of the shutter is detrimental for an economical system. A frame transfer CCD structure does not require a shutter and can have very high fill factors but can be susceptible to creating image smear since the exposure is controlled by shifting the entire image into a light protected storage area after the integration time period has elapsed. The shifting process takes place one line at a time so the last line into storage has been shifted through every other line position on the image. The shift is not instantaneous so some new charge is collected with every shift until the light protected area is reached. This smear effect is not usually a problem if the image transfer time is a small fraction of the total integration time. Where system cost is an issue, high frame rates are not possible and the effects of smear must be considered.
Additionally, with all CCD elements, it is not possible to address an individual pixel for read out. If the object of interest only occupies a small portion of the image, it is necessary to read out the entire image before the object can be analyzed. The lack of sub-array read out capability imposes a speed penalty on the system.
As evident from the above discussion, the use of a CCD for an image sensor puts some burdens on the wheel alignment system in terms of electronic design considerations. The result of these restrictions is increased system cost and loss of flexibility.
There are other imagers on the market that address some of these problems. For example, CMOS imagers are available that address some of the blooming problems.
Current CMOS imagers generally have two types of electronic shutter control or exposure control. Both of these types specify the exposure for the entire array with the goal of trying to keep the exposure uniform across the array. The first type is commonly referred to as a snap shot mode or still mode. This mode is generally used to acquire a single image at a time. In this mode the imager array is initially reset (pixel wells are cleared of all charge), then the imager array is allowed to integrate light and accumulate charge for a period of time, then the imager array is clocked out.
The disadvantage of these snap shot modes is the array is active and still integrating while being clocked out. This can cause a general intensity gradient across the image which is undesirable. At additional cost and complexity a mechanical shutter can be used to block the light during the clock out stage. Alternately a light source (flash) can be turned on during the integration phase to increase the light level, then turned off during the clock out phase to reduce the effect of integrating light during the clock out phase.
The second general exposure mode is referred to as video mode, or rotating shutter, or continuous shutter. This mode is generally used for continuous video applications. In this mode the exposure is controlled on a row-by-row basis. In order to describe how these modes work, it is helpful to first define the term “row time” which is the time required to clock out a single row. By way of illustration, assume an imager specifies its exposure in increments of row times. First the entire imager array is placed in a reset state. Then starting at the top of the imager, the first row is allowed to integrate for a specified number of row times. Meanwhile, after one row time has expired, the second row is allowed to start integrating. After another row time has expired the third row is allow to start integrating and this continues down the array. Now when the specified integration time has expired for the first row, it is then clocked out, and then reset. Immediately thereafter the second row is clocked out then reset and this continues down the array where the operation then wraps back around to the top and continues. One way to think of this mode is to visualize an exposure window (where the array is integrating) that travels from the top of the array to the bottom and then rotates around back to the top, and the row following the exposure window is clocked out and reset until the exposure window wraps back around.
The rotating shutter mode has an advantage over snap shot mode in that the pixels are only exposed for the specified integration time so there is not additional unwanted light as with snap shot mode. The disadvantage of rotating shutter mode is if something moves in the scene, there will be a discontinuity in the image because the bottom pixels are being integrated at a different time than the top pixels. Also there is a maximum limit to the integration time that is dependent on the size of the image being acquired and the desired frame rate. The integration time can be expanded by specifying a larger image or by adding more blanking time between frames, but this slows down the overall frame rate. The other disadvantage of the rotation shutter is when a camera wants to acquire a single frame, the software has to wait for the next top of frame before acquiring the image. Additionally if using external lighting, the lights have to be turned on when the first row is being integrated and left on until the entire array has been clocked out. This can be difficult to coordinate and typically the lights are on longer than would be required for a similar exposure in snap shot mode. The brighter lights can be irritating to the user of the camera system.
Recently, a paper by Acosta-Serafini, P. M.; Masaki, I.; Sodini, C. G. (“A ⅓” VGA Linear Wide Dynamic Range CMOS Image Sensor Implementing a Predictive Multiple Sampling Algorithm with Overlapping Integration Intervals”, IEEE 2003 Custom Integrated Circuits Conference, pp. 485ff.) described a method where the integration time of a pixel or group of pixels can be controlled individually. This is quite different from what is described above. The paper describes a technique for finding and controlling optimum integration time at each pixel site. The goal of the paper was to produce a high dynamic range imager. The technique for controlling the exposure at individual pixel sites is to basically hold some pixels in a reset state longer than others. So the overall exposure time is dictated by the pixel that requires the longest exposure, the other pixels are controlled to limit their exposure time by holding them in reset longer so their integration time is a fraction of the overall time.
In the paper, the total integration time is divided into integration slots of different duration, which are temporally arranged to have a common ending with the longest integration slot matching the total integration time. At the (potential) beginning of each integration slot (in the total integration interval), a pixel check occurs for each pixel. If saturation is predicted, the pixel is reset and allowed to integrate for a shorter period of time (the next integration slot). If saturation is predicted not to happen, the pixel is allowed to integrate for the remainder of the current integration slot. For any given pixel that has predicted saturation (i.e., over-exposure by the end of the total integration interval), the pixel check is repeated at the start of the next integration slot. So a given pixel can be reset numerous times during the total integration interval, if needed to keep that pixel from over-saturating.
The pixel site and A/D portion of the imaging device in said paper basically has a fixed dynamic range. Adjusting the exposures at individual exposure sites can increase the effective dynamic range of the device. In this fashion the resolution of the integration time is added to the resolution of the A/D converter. As a result, dim areas of the scene can be amplified by increasing the integration time so the full resolution of the pixel and A/D converter can be used. For bright areas of the scene, the integration time can be likewise reduced.
This technique could also be applied to produce an apparent logarithmic response to the imager. The pixel site response is basically linear but the brightness and integration time values for each pixel can be directly mapped to single logarithmic brightness values, which more closely emulates the human eye. Of course, other mathematical responses (such as a polynominal curve response) can also be implemented in a similar manner using the present invention.
In addition to the system disclosed in said paper, there is at least one imager with wide dynamic range that extends to very dim images. The imager sold under the trade designation HDRC by IMS Vision of Stuttgart, Germany has such capabilities. That imager is said to be capable of sensing over illumination levels of between 0.001 lux and 500,000 lux. It is a CMOS imager recommended for e.g., vehicle mounted cameras, welding and furnace monitoring, surveillance, vehicle night vision sensors, and security cameras.
Outside the CMOS area, there are other potential approaches. For example, there is a CCD chip sold by Fuji under the trade designation SuperCCD SR that has two separate photo detectors at each pixel site. One photo detector has much lower sensitivity than the other. This structure provides a way to discriminate between light and dark areas of the scene, without unnecessarily losing detail in either area.
Conventionally, image sensor wheel alignment systems use retroreflective targets mounted to the wheel tire assemblies, in combination with strobe lighting surrounding the imagers to help identify the regions of interest in the scene and to measure the relevant orientations of the targets. Retroreflective targets are, however, relatively expensive, while the strobe lighting can prove aggravating to the technician using the system.
There exists, therefore, room for improvement.